Operations of prior art vertical shaft bioreactors for waste water treatment rely on the liquid depth in the reactor to improve oxygen solubility and, thereby, oxygen transfer rate based on the principles of Henry's Law. Such prior art reactors have features that are different from the others and wherein each claims to improve overall reactor and process performance.
Bioreactors of use in the Biohoch Process (Germany), for instance, have reactors that are approximately 20 m deep and, thus, of optimum depth for oxygen transfer per unit of energy employed. Biohoch reactors, however, are completely back-mixed reactors wherein the substrate concentration and, therefore, the rate of reaction is the same throughout the reactor.
Multi Reactors (Holland) use a simulated plug flow configuration where a number of back-mixed cells are placed in a vertical column, generally, totalling about 20 m-50 m in depth. This arrangement allows the reaction rate to change with depth, i.e. time in the reactor. However, the highest oxygen concentration occurs at the bottom of the column wherein the reaction rate is lowest.
Reactors known as the ICI "Deep Shaft" bioreactors, generally, comprise a combination of plug flow and back-mix reactors achieved by using a high internal recycle flow in a very deep reactor. The shaft of the ICI process is 40 m or greater in depth and features a down-flow section (downcomer) having liquor flowing at sufficient velocity to cause undissolved aeration gas to be entrained in, and dragged down, to the bottom of the reactor by liquid flow before returning to the surface.
In general, the oxygen transfer capability of Deep Shaft bioreactors and processes far exceeds the uptake rate required in a typical bioreactor. The generous capacity of a Deep Shaft bioreactor process to effect dissolution of gas into solution provides gas saturation levels sufficient to achieve solids/liquid separation by flotation. Deep Tanks and Multi Reactors have also had moderate success using flotation separation, but at relatively low mixed liquor/solids concentrations.
Long vertical shaft bioreactor systems suitable for the treatment of waste water by activated sludge processes are known and disclosed, as for example, in U.S. Pat. No. 4,279,754 issued to Pollock.
A vertical shaft bioreactor system for the treatment of waste water, typically, comprises a bioreactor, a solid/liquid separator and intervening apparatus in communication with the bioreactor and separator. As fully described in U.S. Pat. No. 4,279,754, such bioreactors essentially comprise a circulatory system which includes at least two substantially vertical side-by-side chambers in communication with each other at their upper and lower ends, with their upper ends being connected through a surface basin. The waste water for treatment is caused to circulate repeatedly between the downflow chamber, termed the downcomer and the upflow chamber, termed the riser. Normally, the waste-containing liquor, referred to as "mixed liquor", is driven through the circulating system by injection of an oxygen-containing gas, usually air, into one or both of the chambers. Typically, in a 500 ft. deep reactor, air injection at a pressure of 100 pounds per square inch is at a depth of about 200 ft. At start-up of the bioreactor, a mixture of air and influent waste water is injected into the riser in the nature of an air lift pump. However, once circulation of the mixed liquor begins, air injection can be also into the downcomer. The fluid in the downcomer has a higher density than the liquid-bubble mixture of the riser and thereby provides a sufficient lifting force to maintain circulation. Usually the surface basin is fitted with a baffle to force mixed liquor at the top of the riser to traverse a major part of the basin to effect release of spent gas before the liquor again descends in the downcomer for further treatment.
Influent waste water is introduced at depth into the riser chamber through an upwardly directed outlet arm of an influent conduit. An oxygen-containing gas, usually air, is injected into the influent liquor in the outlet arm of the influent liquor conduit. In addition to oxygenating the waste liquor, the injected gas acts to create an air lift pump which draws the influent waste into the bioreactor riser. Effluent liquor is withdrawn from the riser through an effluent liquor conduit having its inlet located in the riser at a point below the outlet of the influent liquor conduit. During operation of the bioreactor the flow of influent liquor to and effluent liquor from the bioreactor are controlled in response to changes in level of liquid in the connecting upper basin.
The injected oxygen-containing gas dissolves in the mixed liquor as the liquor descends in the downcomer to regions of greater hydrostatic pressure. This dissolved oxygen constitutes the principal reactant in the biochemical degradation of the waste. As the circulating mixed liquor ascends in the riser to regions of lower hydrostatic pressure the dissolved gas separates and forms bubbles. When the liquid/bubble mixture from the riser enters the basin, gas disengagement occurs.
Reaction between waste, dissolved oxygen, nutrients and biomass substantially takes place during circulation through the downcomer, riser and basin bioreactor system. The products of the reaction are carbon dioxide and additional biomass, which in combination with unreacted solid material present in the influent waste water, forms a sludge.
Autothermal thermophilic aerobic digestion, herein termed ATAD, is a waste water sludge treatment process whereby pre-thickened influent sludge is digested, stabilized and pasteurized for pathogen control. ATAD has been studied and developed since the 1960's, and currently is successfully implemented in several European countries and Canada. ATAD systems are normally two to three stage aerobic processes that operate under themophilic temperature regimes (45.degree. C.-60.degree. C.) without supplemental heat.
In a conventional ATAD system, the reaction tanks, normally two or three, are connected in series, and raw thickened sludge is batch fed into the first reactor. Typically, it is then aerated and mixed with an impeller or venturi type mixer/aerator. Since the incoming raw sludge is at a temperature of about 10.degree. C. to 20.degree. C., the micro-organisms present in the influent are primarily upper psycrophils and lower mesophils. During the initial aeration start up, the mesophils consume the organic materials herein termed the "organics" in the sludge and generate enough heat to raise the temperature to the lower thermophilic range. Subsequently, thermophils begin to dominate the culture, further oxidize the biomass and raise the temperature of the liquor to about 50.degree. C. to 55.degree. C. This first reaction substantially stabilizes the sludge and represents about 60% of the bioxidation possible in a two-stage system. The first reactor does not completely pasteurize the sludge.
Microbes that biologically function in the temperature range of 10.degree. C.-20.degree. C. are "psycrophilic", those that operate in the range of 20.degree. C.-42.degree. C. are "mesophilic", while those that dominate in the temperature range of 45.degree. C.-60.degree. C. are "thermophilic". It is understood that there is no clear operating boundary between mesophils and thermophils, since some of each species exist at both mesophilic and thermophilic temperatures.
In "waste water treatment", nutrients and organics are removed from the waste water and a fraction is converted to new cell matter (biomass) or sludge. The resulting water product can then be recycled or discharged. The sludge produced in waste water treatment can be recycled and/or digested prior to disposal. In "sludge digestion" the sludge is stabilized i.e. the cell mass is reduced and gases are released and pasteurized (pathenogenic reduction). The product water can also be recycled or discharged. Waste water treatment usually takes place in less than one day, while digestion takes several days.
The term "autothermophilic" describes thermophilic processes that generate heat energy.
Meosphils will generally produce more sludge than thermophils and they can oxidize ammonia to nitrate, while thermophilic microbes do not. Mesophils are robust and can withstand shock loads more effectively than thermophils, but they are not as efficient at biodegrading some refractory compounds as can thermophils. Thermophilic processes are favoured in aerobic digestion since the consumption of digestible biomass creates only a small amount of new cell mass resulting in overall volatile suspended solids (VSS) reduction. This, thus, leaves more of the biodegradable portion to be converted to heat.
In a conventional ATAD Fuchs two reactor system and a 5 day retention time, aeration is stopped each day for one hour and approximately 2/5 of the contents of Reactor II is removed. This allows 2/5 of Reactor I volume to be transferred to Reactor 11 and Reactor 1 topped up with new raw sludge. Aeration is resumed for 23 hours and the cycle is repeated. Reactor 1, therefore, retains 3/5 of its volume, heat and thermophilic culture in order to thermophilically process incoming waste. Reactor 11 receives 2/5 of Reactor 1 sludge containing a culture of thermophils and, very importantly, the latent heat in the transferred fluid. This transfer allows Reactor 11 to operate at higher thermophilic and lower stenothermophilic temperatures (55.degree. C.-65.degree. C.), even though there is a reduced amount of organic matter available for bioxidation and heat generation.
Beneficially higher temperatures in Reactor 11 are prevented due to the diminishing amount of biodegradable organics, the loss of heat in the aeration off-gas, the transfer of heat to the treated sludge leaving the tank, and the loss of heat through the reactor wall.
A heat exchange may be provided to recover heat from the effluent of Reactor 11 to heat up the influent of Reactor 1. This approach has limited value since oxygen solubility decreases with increasing temperature.
The effect of temperature on required residence time in the reactor for pathogen control in a Class A (EPA CFR40 Reg. 503) sludge is of critical importance. At 55.degree. C., the required residence time is about 72 hours, whereas at 65.degree. C. the residence time is about 21/2 hours. The U.S. EPA 503 regulation and the European (German) ATAD set standards coverage at about 67.degree. C., which is the practical maximum for existing ATAD processes. Pre-stage technology is not a true ATAD process since it is externally pre-heated, i.e. generated reaction heat is recycled to influent sludge as opposed to thermophil recycle as in ATAD.
Early attempts using ATAD technology with only one tank were moderately successful, but long residence times of 10-15 days were required for pathogen kill. Batch feeding of the single tank on a daily basis reinoculated the treated sludge, which reduces the quality to an EPA Class B sludge. The two tank system evolved to control reinoculation, by emptying part of Reactor 11, transferring part of Reactor 1 into Reactor 11, then topping up Reactor 1. Experience in Europe recommends the time between batch transfers should be limited to once per day, depending on Reactor temperatures, and that the total processing time should be at least 5 days and equally split between the two reactors.
The empirical five day minimum has more to do with the biodegradation of organics than the pasteurization of sludge. This indicates that biodegradation and not pasteurization or reinoculation time must govern the design of bioreactor processes and apparatus.
If the biodegradation rate could be accelerated to match the time for pasteurization, then residence time could be as low as two days and equivalent to the reinoculation time of current ATAD systems. However, a two day retention in a conventional ATAD would require 100% of Reactor 1 to be transferred to Reactor 11 each day. By doing this, all the heat and thermophils would be lost in Reactor 1 and the process reaction would cease.
There is, thus, a need to provide improved apparatus and processes to provide accelerated and more efficacious bioxidation of biomass in waste water and sludge digestion.